Re: [PATCH v2] bluetooth/l2cap: sync sock recv cb and release

Luiz Augusto von Dentz
2024-06-20T16:53:34Z

Hi Edward,

On Fri, Jun 14, 2024 at 9:46 PM Edward Adam Davis <eadavis@qq.com> wrote:
>
> The problem occurs between the system call to close the sock and hci_rx_work,
> where the former releases the sock and the latter accesses it without lock protection.
>
> CPU0 CPU1
> ---- ----
> sock_close hci_rx_work
> l2cap_sock_release hci_acldata_packet
> l2cap_sock_kill l2cap_recv_frame
> sk_free l2cap_conless_channel
> l2cap_sock_recv_cb
>
> If hci_rx_work processes the data that needs to be received before the sock is
> closed, then everything is normal; Otherwise, the work thread may access the
> released sock when receiving data.
>
> Add a chan mutex in the rx callback of the sock to achieve synchronization between
> the sock release and recv cb.
>
> Sock is dead, so set chan data to NULL, avoid others use invalid sock pointer.
>
> Reported-and-tested-by: syzbot+b7f6f8c9303466e16c8a@syzkaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> ---
> net/bluetooth/l2cap_sock.c | 25 ++++++++++++++++++++++---
> 1 file changed, 22 insertions(+), 3 deletions(-)
>
> diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
> index 6db60946c627..f45cdf9bc985 100644
> --- a/net/bluetooth/l2cap_sock.c
> +++ b/net/bluetooth/l2cap_sock.c
> @@ -1239,6 +1239,10 @@ static void l2cap_sock_kill(struct sock *sk)
>
> BT_DBG("sk %p state %s", sk, state_to_string(sk->sk_state));
>
> + /* Sock is dead, so set chan data to NULL, avoid other task use invalid
> + * sock pointer.
> + */
> + l2cap_pi(sk)->chan->data = NULL;
> /* Kill poor orphan */
>
> l2cap_chan_put(l2cap_pi(sk)->chan);
> @@ -1481,12 +1485,25 @@ static struct l2cap_chan *l2cap_sock_new_connection_cb(struct l2cap_chan *chan)
>
> static int l2cap_sock_recv_cb(struct l2cap_chan *chan, struct sk_buff *skb)
> {
> - struct sock *sk = chan->data;
> - struct l2cap_pinfo *pi = l2cap_pi(sk);
> + struct sock *sk;
> + struct l2cap_pinfo *pi;
> int err;
>
> - lock_sock(sk);
> + /* To avoid race with sock_release, a chan lock needs to be added here
> + * to synchronize the sock.
> + */
> + l2cap_chan_hold(chan);
> + l2cap_chan_lock(chan);
> + sk = chan->data;
>
> + if (!sk) {
> + l2cap_chan_unlock(chan);
> + l2cap_chan_put(chan);
> + return -ENXIO;
> + }
> +
> + pi = l2cap_pi(sk);
> + lock_sock(sk);
> if (chan->mode == L2CAP_MODE_ERTM && !list_empty(&pi->rx_busy)) {
> err = -ENOMEM;
> goto done;
> @@ -1535,6 +1552,8 @@ static int l2cap_sock_recv_cb(struct l2cap_chan *chan, struct sk_buff *skb)
>
> done:
> release_sock(sk);
> + l2cap_chan_unlock(chan);
> + l2cap_chan_put(chan);
>
> return err;
> }
> --
> 2.43.0

Looks like this was never really tested properly:

============================================
WARNING: possible recursive locking detected
6.10.0-rc3-g4029dba6b6f1 #6823 Not tainted
--------------------------------------------
kworker/u5:0/35 is trying to acquire lock:
ffff888002ec2510 (&chan->lock#2/1){+.+.}-{3:3}, at:
l2cap_sock_recv_cb+0x44/0x1e0

but task is already holding lock:
ffff888002ec2510 (&chan->lock#2/1){+.+.}-{3:3}, at:
l2cap_get_chan_by_scid+0xaf/0xd0

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&chan->lock#2/1);
lock(&chan->lock#2/1);

*** DEADLOCK ***

May be due to missing lock nesting notation

3 locks held by kworker/u5:0/35:
#0: ffff888002b8a940 ((wq_completion)hci0#2){+.+.}-{0:0}, at:
process_one_work+0x750/0x930
#1: ffff888002c67dd0 ((work_completion)(&hdev->rx_work)){+.+.}-{0:0},
at: process_one_work+0x44e/0x930
#2: ffff888002ec2510 (&chan->lock#2/1){+.+.}-{3:3}, at:
l2cap_get_chan_by_scid+0xaf/0xd0

l2cap_sock_recv_cb is assumed to be called with the chan_lock held so
perhaps we can just do:

sk = chan->data;
if (!sk)
return -ENXIO;

--
Luiz Augusto von Dentz